Einstein's Train
This version of the Einstein's Train scenario is based on an acoount in Wikipedia. It seeks to explain how differences in simultaneity arise:
- A strobe light mounted in the middle of a railway carriage sends out a single pulse towards the front and the back of the carriage.
- To an observer on the train the ends of the carriage are an equal distance from the source of the light, and the light approaches both at the same speed. So the light reaches the front and the back of the carriage simultaneously.
- To an observer on the platform, the light is moving towards the back and front of the carriage at equal speeds, but because the carriage is moving, the back of the carriage is approaching the light, whilst the front is receding. So the light travelling to the back of the carriage has less distance to travel than the light travelling to the front. So the light reaches the back of the carriage before the front.
- Hence events which are simultaneous to the observer on the train are not simultaneous to the observer at the station.
That's a summary of the explanation given in Wikipedia, and it seems to suggest that the difference in simultaneity is a real, measurable, occurrence. However, there are reasons to question this view:
- The difference in simultaneity has never actually been empirically tested.
- It only arises between events that are not, and cannot, be causally related. So it may be impossible to prove whether it exsts or not anyway. I.e. How do you prove whether two unrelated events happened simultaneously or not, particularly when viewed from different frames of reference?
- Dr John Field, who is a former researcher at CERN, has reworked Einstein's derivation of the Lorentz transformation. His work, which is summarised here, suggests that the well known, and well proven, differences in the elapse of time between events arise without a difference in simultaneity.
This leads us to question whether the difference in simultaneity is a real phenomenon? We pursue that question in the Two Towers example. However, we have another myth to consider before we reach that one...
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